1. Field
One embodiment of the present invention relates to an optical integrator, illumination optical apparatus, exposure apparatus, and device manufacturing method. More particularly, the present invention relates to an optical integrator suitably applicable to illumination optical apparatus in exposure apparatus used for manufacturing such devices as semiconductor devices, imaging devices, liquid-crystal display devices, and thin-film magnetic heads, for example, by lithography.
2. Description of the Related Art
In a typical exposure apparatus of this type, a beam emitted from a light source is guided into a fly's eye lens as an optical integrator of a wavefront division type, to form a secondary light source consisting of a large number of illuminants on or near a rear focal plane of the fly's eye lens. Beams from the secondary light source are guided through an aperture stop disposed on or near the rear focal plane of the fly's eye lens, to be limited, and they are then incident to a condenser lens. The aperture stop limits the shape or size of the secondary light source to a desired shape or size in accordance with a desired illumination condition (exposure condition).
The beams condensed by the condenser lens illuminate a mask with a predetermined pattern therein in a superimposed manner. Light transmitted by the pattern of the mask travels through a projection optical system to form an image thereof on a wafer. In this manner, the mask pattern is projected (or transferred) onto the wafer to effect exposure thereof. Sine the pattern formed in the mask is a highly integrated pattern, a uniform illuminance distribution must be achieved on the wafer in order to accurately transfer this microscopic pattern onto the wafer.
A conventional technique proposed for improving illuminance uniformity on the wafer is a configuration in which two fly's eye lenses are arranged in tandem in the illumination optical apparatus for illuminating the mask, i.e., a double fly's eye configuration. The double fly's eye configuration described in U.S. Reissued Pat. No. 34,634.
In recent years, attention is being directed toward modified illumination techniques in which the aperture stop located on the exit side of the fly's eye lens has an aperture (light transmitting portion) set in an annular or multi-pole (dipole, quadrupole, or the like) shape to limit the shape of the secondary light source to the annular or multi-pole shape, thereby improving the depth of focus and the resolving power of the projection optical system. In the conventional technology of the double fly's eye configuration, a rectangular illumination field is formed on the entrance surface of the second fly's eye lens by action of the first fly's eye lens (light-source-side fly's eye lens) and the secondary light source of the rectangular shape is similarly formed on or near the rear focal plane of the second fly's eye lens.
For implementing the modified illumination (annular illumination or multi-pole illumination) in this case, the beams from the relatively large rectangular secondary light source fouled by the second fly's eye lens are limited by the aperture stop having the aperture of an annular shape or multi-pole shape. Namely, the conventional technology had the problem that the aperture stop blocked a considerable amount of the beams from the secondary light source in the modified illumination, without contribution to illumination (exposure), and the light-quantity loss at the aperture stop caused reduction in illuminance on the mask and on the wafer and, in turn, reduction in throughput as exposure apparatus.
An object of the invention is to provide an optical integrator capable of keeping down the light-quantity loss, for example, in the modified illumination with illumination optical apparatus. Another object of the present invention is to provide an illumination optical apparatus capable of illuminating an illumination target surface under a desired illumination condition, using the optical integrator capable of keeping down the light-quantity loss in the modified illumination. Still another object of the present invention is to provide an exposure apparatus and device manufacturing method capable of implementing good exposure under a desired illumination condition, using the illumination optical apparatus for illuminating a mask under a desired illumination condition.